Understanding and improving charge transfer pathways between extracted Photosystem I (PSI) protein complexes and electrodes is necessary for the development of low‐cost PSI‐based devices for energy conversion. We incorporated PSI multilayers within porous indium tin oxide (ITO) electrodes and observed a greater mediated photocurrent in comparison to multilayers on planar ITO. First, the mediated electron transfer (MET) pathway in the presence of 2,6‐dichlorophenolindophenol (DCPIP) and ascorbate (AscH) was studied via photochronoamperometry on planar ITO. ITO nanoparticles were then used to fabricate two porous electrode morphologies; mesoporous (20–100 nm pores) and macroporous (5 μm pores). PSI multilayers within macroporous ITO cathodes produced 42±5 μA cm−2 of photocurrent, three times the photocurrent produced by mesoporous ITO. Additionally, macroporous cathodes are able to utilize twice as much active surface area, when compared to mesoporous cathodes. Our findings show that MET within PSI multilayers is greater in 5 μm macropores than mesoporous ITO due to both an increase in electrode surface area and the location of PSI complexes within the pores. Improving MET in PSI‐based bioelectrodes has applications including improving the total charge transfer achieved in PSI‐based photoelectrochemical cells or even incorporation in bio‐photocatalytic cells.
The photosynthetic protein complex, photosystem I (PSI), can be photoexcited with a quantum efficiency approaching unity and can be integrated into solar energy conversion devices as the photoactive electrode. The incorporation of PSI into conducting polymer frameworks allows for improved conductivity and orientational control in the photoactive layer. Polyviologens are a unique class of organic polycationic polymers that can rapidly accept electrons from a primary donor such as photoexcited PSI and subsequently can donate them to a secondary acceptor. Monomeric viologens, such as methyl viologen, have been widely used as diffusible mediators in wet PSI-based photoelectrochemical cells on the basis of their suitable redox potentials for accepting electrons. Polyviologens possess similar electronic properties to their monomers with the added advantage that they can shuttle electrons in the solid state. Depositing polyviologen directly onto a film of PSI protein results in significant photocurrent enhancement, which confirms its role as an electron-transport material. The polymer film not only improves the photocurrent by aiding the electron transfer but also helps preserve the protein film underneath. The composite polymer–PSI assembly enhances the charge-shuttling processes from individual protein molecules within the PSI multilayer, greatly reducing charge-transfer resistances. The resulting PSI-based solid-state platform demonstrates a much higher photocurrent than the corresponding photoelectrochemical cell built using a similar architecture.
An inexpensive, combinatorial method to evaluate an array of metal oxide materials as photocatalysts for solar fuel production utilizing spray pyrolysis is presented. This new approach capitalizes on the inherent properties of spray pyrolysis. We take advantage of the natural lateral gradient produced in a spray cone to fashion four-metal-threeat-a-time compositional triangular patterns on conductive glass substrates from simple nitrate salt precursor solutions. Subsequent annealing produces thin-film electrodes that are readily screened for photochemical activity using a simple laser scanner system. The apparatus employed is constructed from readily available commercial components, making it accessible to a wide number of laboratories. Our method complements other combinatorial methods in that it provides a chemically different environment for the formation of materials that might produce different morphologies and metal oxidation states and it allows for easy evaluation of layered structures, as well single-phase materials, thereby expanding the number of unique materials tested as potential photocatalysts. As a proof of principle, the discovery and optimization of a new Na-doped CuBi 2 O 4 photocatalyst is described.
LaFe x Co (1– x ) O 3 thin films were prepared on fluorine-doped tin oxide conducting glass substrates by spray pyrolysis without any conductive additives and evaluated for their ability to catalyze the oxygen reduction reaction. Onset potential and current density were found to be comparable to platinum, and the resulting crystallite size was on the order of 20 nm. Coordination of the precursor metal ions by citrate was found not to be advantageous. Results from multiple scan linear sweep voltammetry suggest lattice oxide saturation during reduction of oxygen and lattice oxide depletion upon water oxidation. The color of the best-performing composition changes dramatically between 1.2 and −1.15 V versus saturated calomel electrode, so X-ray photoelectron spectra of the fully oxidized and reduced films were compared, demonstrating that cobalt in the film changes oxidation state. Performance of the films as a function of iron-to-cobalt ratio is consistent with what others have reported in the literature, indicating that spray pyrolysis is an efficient method to prepare and evaluate new catalytic materials.
Using a novel hydrothermal synthesis, nitrogen-doped carbon dots were synthesized and shown to exhibit tunable optical and electrochemical properties.
Invited for this month's cover picture is the group of Dr. David E. Cliffel from Vanderbilt University (USA). The Cover Picture shows a Photosystem I (PSI) protein complex converting sunlight into chemical energy through an electron transfer reaction with dichlorophenolindophenol (DCPIP). The PSI is entrapped within a macroporous indium tin oxide (ITO) electrode which leverages its high surface area to produce electrical energy from reacted DCPIP. Read the full text of the Article at 10.1002/celc.201901628
Biohybrid photovoltaics leverage naturally occurring photo-active protein complexes to convert solar energy into electricity; offering a unique route to lowering the cost and environmental impact of solar energy conversion. Photosystem I (PSI), a protein complex found in higher plants, algae, and cyanobacteria, produces photoexcited excitons over a distance of approximately 10 nm with a 1.1 V photopotential. PSI has been utilized in a variety of biohybrid photovoltaics with success; however, improvements in performance are necessary to bring PSI – based devices to a technologically viable state. The electrons and holes produced within a PSI biohybrid photovoltaic must pass through multiple interfaces before being harnessed in an electrical circuit. Therefore, the total current, potential, and efficiency are dependent on losses incurred at each interface and could be vastly improved by optimizing individual electron transfer steps. Many researchers are looking toward direct electron transfer (DET) between the active sites of PSI and electrodes to reduce these losses. Unfortunately, achieving DET is difficult due to the necessity of proper PSI orientation and the close proximity (<1 nm) of the active sites to the electrode material for electron tunneling. Alternatively, mediated electron transfer (MET) can be employed through the use of soluble redox mediators to shuttle charge between the active sites and electrodes. MET can benefit from the use of high surface area electrodes and increased loadings of PSI. Our group has studied both DET and MET within PSI bioelectrodes of various architectures. In this presentation, the incorporation of PSI into porous and translucent indium tin oxide (ITO) cathodes to achieve significant improvements in MET will be highlighted. Additionally, the limitations and challenges of DET, directions for improving MET in porous electrodes, and future prospects for improving charge transfer at PSI – electrode interfaces will be discussed.
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